Conventionally, in an internal combustion engine (hereinafter, refer to as an engine) mounted to a vehicle, it is attempted to determine a combustion state by detecting an ion current generated within a combustion chamber. Specifically, the structure is made such as to detect the ion current on the basis of a fact that the ion current generated in the combustion chamber after an ignition is greater than a threshold level set for detecting, and determine on the basis of the detected ion current whether or not a combustion state is good.
For example, an invention disclosed in Patent Document 1 is structured such as to start a detection of an ion current at a point in time when a starter starts rotating and a fuel injection is started. Further, a characteristic of the ion current is measured on the basis of a time obtained by summing up times at which the detected ion current is greater than a set value, or a time at which the ion current is generated in a period from the ignition to a final point in time when the ion current is greater than the set value, whereby the combustion state is determined.
Patent Document 1: Japanese Unexamined Patent Publication No. 11-107897
In this case, the ion current is measured by detecting an ion current flowing between an inner wall of the combustion chamber and a center electrode of a spark plug, and between the electrodes of the spark plug, on the basis of a matter that a measuring voltage (a bias voltage) for measuring the ion current is applied to the spark plug after an ignition of the spark plug.
In this case, in a state in which a wall surface temperature of the combustion chamber is sufficiently high, the wall surface comes to a state capable of preferably seizing an electron, that is, an ion generated by the combustion, and it is possible to detect a current value of the ion current which accurately reflects the combustion state.
However, the wall surface temperature of the combustion chamber is going to rise little by little while absorbing a heat of a flame in accordance with a repeat of the combustion after an engine start point in time. Further, a current value of the ion current detected between the inner wall of the combustion chamber and the center electrode of the spark plug becomes higher in correspondence to an ascent of the inner wall of the combustion chamber, that is, the wall surface. In other words, since the wall surface temperature is low just after the engine start, it is impossible to sufficiently seize the ion in accordance with the combustion. As a result, even if a normal combustion is generated within the combustion chamber, there appears a tendency that the current value of the ion current detected between the inner wall of the combustion chamber and the center electrode of the spark plug becomes smaller, for example, than that after warm-up of the engine.
Further, if the combustion state is determined on the basis of the ion current even at a time of starting in a predetermined cycle just after the engine start as described in the Patent Document mentioned above, in the similar manner as the other cases except the predetermined cycle, there is determined on the basis of a value of the ion current detected small in spite of a normal combustion, for example, that the combustion state is lowered or a state close to a misfire. A rich state of an air fuel ratio is caused by erroneously carrying out a control for avoiding the reduction of the combustion or the misfire on the basis of the determination mentioned above, and as a result, there is generated a matter that an exhaust emission is unnecessarily increased.